1. Field of the Invention
The present invention relates to devices for varying vehicle tread widths and, more specifically, for varying the tread width on agricultural vehicles.
2. Description of the Related Art
It has been desirable for many years to provide work vehicles, particularly agricultural tractors, with a mechanism for axially displacing their wheels. Such axial displacement, to either narrow or widen the wheel spacing, allows the operator to adapt the tractor to specific conditions, depending on the nature of the task to be performed. For example, the tread width can be changed to allow planting of crops at different row spacings.
Due to the large size and weight of tractor wheels, it can be appreciated that some mechanism normally is used to facilitate this operation. Many mechanisms involve the power of the tractor rotating the axles to literally "screw" the wheels in or out, such as those disclosed in U.S. Pat. Nos. 2,099,194 (Brown); 2,417,139 (Strehlow); 2,829,725 (Wurdelman) and 3,847,441 (Morkoski et al.). Another mechanism uses a plurality of circumferentially spaced fasteners attached to the wheel hub which engage one of a plurality of interior radial grooves provided on the rim, as shown in U.S. Pat. No. 3,771,832 (Sipovik et al.). U.S. Pat. No. 4,317,596 (Lemmon) teaches yet another mechanism, using a pinion gear housed within one of several arc-shaped wedges positioned between an axle and a wheel hub. Although these aforementioned patents and other patents in the related art may fulfill the criteria of axially displacing wheels, the very cost of their designs often limits their actual use.
U.S Pat. No. 2,676,849 (Houck et al.) introduces various embodiments of a tread-varying device which has been put to actual use for many years. The disclosure teaches using intermeshing toothed mechanisms to axially displace the tractor wheels, e.g., a rotatable pinion gear carried by the wheel hub or outer part that intermeshes with a toothed rack on the axle or inner part. With such a device, a farmer can use a tool to rotate the pinion, adjusting the wheel to a predetermined measurement with a minimum amount of physical effort. Even though Houck et al.'s invention performs quite well and is widely used, problems of high stress from torsional forces may arise in the wheel hub about the area of the pinion gear. This can be attributed primarily to the recent increase in the tractor horsepower being transmitted to the final drive axles. In the Houck et al. design, the wheel hub is cast with a cavity in which the pinion gear is received. This cavity gives the wheel hub an aesthetically pleasing appearance and provides for a protective body in which the pinion can be housed, but the removal of the metal to form the cavity may cause high stresses. Thus, elimination of the pinion gear, housed as an integral part of the wheel hub, would greatly reduce manufacturing costs as well as allow the wheel hub to be a symmetrical mass, without the stress-creating cavity.